Need to Know

Hi Laura and Kelly

Here is the explanation of what Electronic Rust Protection is.

What is Rust?

Rust is a type of Corrosion. More specifically, it is the corrosion of iron, or iron compounds, in the presence of water and oxygen. Rust is actually a mixture of Iron Oxides and Hydroxides. This corrosion happens because the oxidation of Iron is energetically favourable. This means it gives off energy when rust forms. So, you don’t need to put energy in, it can happen spontaneously; as we all know it does (or you wouldn’t have to be selling this product in the first place).

To summarize: Rusting happens in 3 stages:

Stage 1: Iron(II) ions (Fe²⁺) are formed in the metal.

Stage 2: Hydroxide ions (OH^-) are formed in the water.

Stage 3: Iron(II) ions and Hydroxide ions react (with oxygen) to form rust.

STAGE 1:

When an iron comes in to contact with water, an electrochemical process starts. On the surface of the metal, iron is oxidised (which means that it loses electrons) to iron(II):

Fe ‑-> Fe²⁺ + 2e^- (This is an Iron(II) ion plus 2 electrons)

STAGE 2:

The electrons that are released travel to the edges of the water. Here there is plenty of dissolved oxygen. The electrons then reduce the oxygen and water (which means that they gain electrons) to hydroxide ions:

4e^- + O₂ + 2H₂O --> 4OH^- (4 electrons plus oxygen plus 2 water molecules become 4 hydroxide ions)

STAGE 3:

The hydroxide ions react with the iron(II) ions and with more dissolved oxygen to form iron oxide.

Fe²⁺ + 2OH^- ‑-> Fe(OH)₂ (This is called iron(II) oxide)

Iron(II) oxide is green in colour but it is almost never seen because it is usually further oxidized by the oxygen. When 4 of these iron oxide molecules react with oxygen it forms the commonly seen hydrated iron oxide.

2Fe(OH)₂ + ½O₂ + H₂O --> 2Fe(OH)₃ (this is 2 molecules of Iron(III) oxide)

This iron(III) oxide is reddish brown in colour and is what you see on cars as rust. Thus, rust is actually just hydrated Iron(III) Oxide.

Why Do Cars Rust Out?

Unlike the oxides of other metals, like Aluminum, rust does not form a protective layer on the metal. You see, when aluminum oxidizes, the oxidation forms a nice, dense, protective film on the metal so that it inhibits further oxidation. Unfortunately, rust doesn’t do this. Rust is typically flaky and porous, and it easily flakes off. In other words, rust is permeable to air and water. The result is that new metal is continually exposed. So, if left unchecked, the iron will rust right through. In fact, if left completely unchecked, a piece of Iron exposed to the elements will eventually be converted entirely into rust. Nice, eh? (Laura, tell GM to make their cars out of stainless steel)

So, How Do We Stop It?

There are many ways that rusting can be prevented. One way is to protect the surface of the metal from contact with oxygen. Often we do this with paint or oil or other coatings. Or we can do this electronically, by taking advantage of the electrochemical nature of the rusting process. By this I mean, remembering how important electrons are to the process.

Oxidation requires electrons. When something rusts, like my Buick Lesabre, it is ultimately because the oxygen was able to take the electrons from the iron in my car. If I could somehow stop this from happening I would stop the rusting. This means I need another supply of electrons readily available. This is called ‘Cathodic protection’. There are 2 ways we can do this. We can use, what is called, a ‘sacrificial anode’, or we can use an ‘impressed current’.

A ‘sacrificial anode’ involves the use of another more active metal (usually a block of metal, which has a higher electroode potential) which more readily gives up it’s electrons. A good example is magnesium. Magnesium has much more negative electrode potential than iron, and so, is used often as a sacrificial anode. This method of using a more reactive metal to corrode in place of iron is often used to protect ships' hulls, and underground steel pipes. Magnesium plates or blocks are usually attached, or connected by wire, to the iron. This supplies the electrons for the reduction of oxygen. The result is that the magnesium oxidizes instead of the iron hull of the ship or the underground pipe. As long as there is a supply of magnesium the iron is protected. As you can imagine, you would have to periodically replace the magnesium plates.

The ‘impressed current’ method, on the other hand, relies on supplying the required electrons from an electrical source like a battery. What it does is force a uniform flow of free electrons (impressed current) throughout the metal of the vehicle. This constant flow of free electrons in the metal prevents the iron from losing electrons, and thus, protects the iron from oxidation. So, NO RUST !

In short,

make sure there is a readily available supply of electrons (and not from the iron atoms) either through a sacrificial anode or an impressed current, and you will prevent the oxidation of the iron.

How’s that for an explanation. I tried to include enough information just in case you have some real ‘wiz’ who wants to know the details. But hopefully you see that the basic process of electronic rust protection is fairly straight forward. Let me know if you need any more info.

Ray